Systems and methods for producing robust chalcogenide optical fibers

ABSTRACT

In one embodiment, a chalcogenide glass optical fiber is produced by forming a billet including a chalcogenide glass mass and a polymer mass in a stacked configuration, heating the billet to a temperature below the melting point of the chalcogenide glass, extruding the billet in the ambient environment to form a preform rod having a chalcogenide glass core and a polymer jacket, and drawing the preform rod.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation of U.S. patent application Ser. No.14/398,548, filed Nov. 3, 2014, which is the 35 U.S.C. § 371 nationalstage of, and claims priority to and the benefit of, PCT applicationPCT/US2013/039489, filed May 3, 2013, which claims priority to and thebenefit of U.S. Provisional Application No. 61/642,202, filed on May 3,2012, herein incorporated by reference in its entirety.

NOTICE OF GOVERNMENT-SPONSORED RESEARCH

This invention was made with Government support under contract/grantnumber ECCS-1002295, awarded by the National Science Foundation (NSF).The Government has certain rights in the invention.

BACKGROUND

There is currently great interest in optical fibers that can be used inmid-infrared (MIR) applications, such as the delivery of quantum cascadelasers. While silica optical fibers are commonplace and easy to obtain,such fibers have a limited window of wavelengths at which they aretransparent and are highly absorbing of MIR light. Chalcogenide (ChG)glasses, on the other hand, are transparent across the entire infrared(IR) spectrum. While ChG can be used to produce optical fibers, ChG isextremely brittle and it is therefore difficult to produce robust ChGfibers.

Although silica fibers are made by drawing a preform in the ambientenvironment, ChG fibers cannot be produced in this manner because theyare sensitive to the environment and oxidize easily. Instead, ChG fibersare typically produced by melting two ChG glasses (one for the core andone for the cladding) in a protected environment and drawing a fiberfrom a nozzle in a manner in which one glass surrounds the other. Oncethe fiber has been drawn, a thin polymer jacket can be applied. Whilethis manufacturing method is feasible, it is difficult to maintain auniform draw for long lengths (e.g., over 10 m) of the ChG glasses usingthe method because they are so soft during the draw process. Inaddition, it can be difficult to apply the polymer jacket because of thefragility of the ChG glass. Furthermore, because the polymer jacket isthin, the ChG optical fiber end product is quite fragile. Moreover,because of the conditions required to make the fiber, ChG optical fibersare extremely expensive.

From the above discussion, it can be appreciated that it would bedesirable to have a way to produce robust ChG optical fibers withgreater ease.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure may be better understood with reference to thefollowing figures. Matching reference numerals designate correspondingparts throughout the figures, which are not necessarily drawn to scale.

FIG. 1 is a flow diagram of an embodiment of a method for producingrobust chalcogenide optical fibers.

FIG. 2 is an exploded perspective view of a first embodiment of a billetthat can be used to produce robust chalcogenide optical fibers.

FIG. 3 is a perspective view of a second embodiment of a billet that canbe used to produce robust chalcogenide optical fibers.

FIG. 4 is a schematic view of an extrusion system that can be used toproduce robust chalcogenide optical fibers.

FIG. 5 includes images of (a) a vertically-stacked billet, (b) a drawnGGP fiber, (c) and (e) transmission optical micrographs of the fibercross sections, and (d) and (f) reflection micrographs of the core.

FIG. 6 includes images of (a) a hybrid polymer and ChG billet, (b) asection of an extruded preform, (c) a disc cut from the extrudedpreform, and (d) and (e) reflection optical micrographs of the fibercross-section and core, respectively.

FIG. 7 includes images of (a) a 1-mm-diameter fiber tied in a 1-inchdiameter knot, (b) a plot of transmission versus time for 10 fibersafter bending the fiber with D=0.5 inch bend diameter (the thick curveis the average of all the measurements), (c) a 2-kg weight hanging froma 5-cm-long fiber (insets to the right show the hanging mechanism), and(d) a robust multi-material taper (inset above is a micrograph of thetaper center).

FIG. 8 includes images of characterization of GGP tapers ((a)-(c)) andGP tapers ((d)-(f)). Figs. (a) and (d) are SEM micrographs of the crosssections, Figs. (b) and (e) are white-light images, and Figs. (c) and(f) are 1.55-μm laser light near-field intensity images.

FIG. 9 is a plot of the measured and calculated optical transmission at1.55 μm for GGP and GP tapers with different d_(min) normalized withrespect to the untapered fiber with d_(o)=10 μm. The dashed verticalline corresponds to d_(min)=1.4 μm used in FIGS. 8(a)-(f).

DETAILED DESCRIPTION

As described above, it would be desirable to have a way to producerobust chalcogenide (ChG) optical fibers with greater ease. Disclosedherein are systems and methods for producing robust ChG optical fibers.In one embodiment, a billet is formed that comprises one or more ChGglass masses and a polymer mass that are arranged in a stackedconfiguration. In some embodiments, the ChG glass and polymer massescomprise independent cylindrical discs that are stacked in apredetermined order. The billet is heated to a temperature that enablesviscous flow of the ChG glass but that does not melt the glass. Once thedesired temperature is reached, the billet is extruded within theambient environment to form an extruded rod that has a ChG core and apolymer jacket that protects the ChG from the environment and enablesfurther processing. The rod can then be drawn to obtain an optical fiberhaving a desired diameter. Optionally, further polymer material can beapplied to the rod or the fiber to form a relatively thick jacket thatprovides greater mechanical protection to the ChG glass.

In the following disclosure, various specific embodiments are described.It is to be understood that those embodiments are exampleimplementations of the disclosed inventions and that alternativeembodiments are possible. All such embodiments are intended to fallwithin the scope of this disclosure.

A one-step, multi-material extrusion fabrication approach for theproduction of ChG optical fibers is described below. With this approach,composite ChG/polymer preforms are produced that can be drawn intorobust fibers. A billet comprising a polymer and ChG is extruded into apreform having a built-in polymer jacket. The polymer does notparticipate in the optical functionality of the fiber, which is dictatedby the ChG alone. The resulting optical fibers can also be used to formrobust, high-index-contrast, submicron-core-diameter tapers suitable fornonlinear optical applications without removing the polymer.

FIG. 1 is a flow diagram that describes an embodiment of a method forproducing a robust ChG optical fiber. Beginning with block 10 of thefigure, a billet is formed that comprises one or more ChG glass massesand a polymer mass in a stacked arrangement. FIG. 2 illustrates anexample billet 30 in an exploded view. In the example of FIG. 2, thebillet 30 includes a first ChG glass mass 32, a second ChG glass mass34, and a polymer mass 36 that together form a stack 38 of masses. Thefirst ChG glass mass 32 is at the top of the stack 38, the second ChGglass mass 32 is in the middle of the stack, and the polymer mass 36 ispositioned at the bottom of the stack. The order of the masses dictatesthe positions that the materials of the billet 30 will occupy in theoptical fiber. Specifically, the top mass will form a core of the fiber,the middle mass will form an intermediate layer (cladding) of the fiber,and the bottom mass will form an outer layer (jacket) of the fiber.

In some embodiments, the first ChG glass mass 32 can comprise As₂Se₃ orAs₂Se_(1.5)S_(1.5) and the second ChG glass mass 34 can comprise As₂S₃.In some embodiments, the polymer comprises a thermally compatiblethermoplastic polymer such as polyethersulfone (PES), polyetherimide(PEI), polysulfone, polycarbonate, or cyclo-olefin polymer or copolymer.

In the example of FIG. 2, each mass is formed as a cylindrical masshaving a circular cross-section. In some embodiments, the diameter ofthe masses is approximately 25 to 50 mm. The relative heights of themasses dictate the nature of the optical fiber that will be formed.Specifically, the ratios between the heights of the masses will dictatethe ratios of the materials across the cross-section of the fiber.Accordingly, the heights of the masses can be varied to vary thecross-sectional ratios of the fiber materials. In some embodiments, thefirst ChG glass mass 32 is approximately 5 to 30 mm tall, the second ChGglass mass 34 is approximately 5 to 30 mm tall, and the polymer mass 36is approximately 5 to 30 mm tall.

FIG. 3 illustrates an alternative billet 40. The billet 40 also includesa first ChG glass mass 42, a second ChG glass mass 44, and a polymermass 46 that together form a stack 48 of masses. The masses can be madeof the same materials described above in relation to FIG. 2. In theembodiment of FIG. 3, however, the polymer mass 46 includes a void 50 inwhich the ChG masses 42, 44 are disposed in a stacked configuration inwhich the ChG glasses are surrounded on all sides by polymer andtherefore will not come into contact with the walls of an extruder. Withthis configuration, the first ChG glass mass 42 (top mass) will form thecore of the fiber, the second ChG mass 44 (middle mass) will form theintermediate layer (cladding) of the fiber, and the polymer mass 46(bottom mass) will form the outer layer (jacket) of the fiber.

As in the embodiment of FIG. 2, each mass in the billet 40 is formed asa cylindrical mass having a circular cross-section. In some embodiments,the outer diameter of the polymer masses is approximately 25 to 50 mmand the diameter of the ChG glass masses 42, 44 is approximately 10 to25 mm. In some embodiments, the first ChG glass mass 42 is approximately5 to 15 mm tall, the second ChG glass mass 44 is approximately 5 to 15mm tall, and the polymer mass 46 is approximately 25 to 60 mm tall.

With reference back to FIG. 1, the billet, irrespective of itsparticular form, can be placed within an extrusion chamber of anextrusion system, as indicated in block 12. FIG. 4 illustrates anexample of an extrusion system 60, which can be positioned within afurnace (not shown). The system 60 includes an extrusion chamber 62 inwhich an example billet 64 has been placed. In embodiments in which thebillet 64 is cylindrical, the extrusion chamber 62 can be formed by acylindrical sleeve 66 that has an inner diameter of approximately 30 to46 mm. The sleeve 66 includes a circular die 68 through which the billet64 is extruded. In some embodiments, the die 68 has an inner diameter ofapproximately 6 to 20 mm. The extrusion system 60 further includes apiston 70 that is used to drive the billet material through the die.

With reference to block 14 of FIG. 1, the billet is next heated to atemperature that is higher than the softening temperature of the ChGglass but lower than the melting point of the glass. In someembodiments, the billet is heated to a temperature of approximately 180to 330° C. Once the billet is heated to that temperature, viscous flowis possible. Therefore, as indicated in block 16, the billet can beextruded to form a preform rod that comprises a ChG core and a polymerjacket. In embodiments in which the billet comprises two masses of ChGglass, the rod will further comprise a ChG glass cladding positionedbetween the core and the jacket. In some embodiments, the extrusion canbe performed using approximately 90 to 1000 pounds of force to extrudethe billet at a rate of approximately 0.3 to 0.7 mm/minute. Suchextrusion under pressure enables the use of lower temperatures andhigher viscosities as compared to fiber drawing, thereby reducing glasscrystallization. The outer diameter of the rod and the diameter of thecore will depend upon the inner diameter of the die of the extrusionsystem. In some embodiments, the preform rod has an outer diameter ofapproximately 2 to 20 mm and a core diameter of approximately 500 to6000 μm.

At this point, further polymer material can be added to the preform, ifdesired, as indicated in block 18. Such additional material providesfurther mechanical stability to the preform that enables processing ofthe rod into an optical fiber. In some embodiments, a thin film ofpolymer can be rolled onto the rod or the rod can be inserted into apolymer tube.

Next, with reference to block 20, the preform rod can be drawn to forman optical fiber having the desired diameter. In some embodiments, therod can be drawn to form an optical fiber having an outer diameter ofapproximately 0.5 to 2 mm, a core diameter of approximately 2 to 500 μm,a cladding having a thickness of approximately 1 to 500 μm, and a jackethaving a thickness of approximately 200 to 1000 μm. The jacket can becalled a “built-in” jacket because the jacket material was added to thefiber (or to the rod) prior to the final drawing.

ChG optical fibers were fabricated using the methodologies describedabove. A preform was made with a first ChG glass for the core and asecond ChG glass for the cladding. A vertically-stacked billet wasformed comprising polished discs placed atop each other as shown in FIG.5(a). The top disc comprised one of As₂Se₃ (G1) and As₂Se_(1.5)S_(1.5)(G2), the middle disc comprised As₂S₃ (G3), and the bottom disccomprised PES or PEI(P). The three glasses had measured indices of2.904, 2.743, and 2.472 at 1.55 μm, respectively. The largeindex-contrast between the ChG glasses was chosen in order to test thelimits of the extrusion process and to produce high-index-contrastnano-tapers described below.

The billet was heated and extruded to form an extruded preform thatcomprised nested shells with a ChG core, a ChG cladding, and a polymerjacket. This structure is referred to hereafter as GGP. The polymerprotected the ChG from coming in contact with the die during extrusionor subsequently with the ambient environment. No separation between thelayers in the preforms or in the subsequently drawn fibers was observed.The relatively large thermal expansion coefficient of the polymereliminated small gaps from the preform that inevitably exist atinterfaces in the billet.

Each preform was then drawn into a cane and a 10-cm long section of thecane was inserted into a polymer tube, which in turn was drawn intoapproximately 100 m of continuous, robust, 1-mm outer diameter, 10-μmcore diameter fiber shown in FIG. 5(b). Cross-sections of the two GGPfibers, G1-G3-PEI and G2-G3-PES, are shown in FIGS. 5(c) and (d) andFIGS. 5(e) and (f), respectively. The ChG in the latter two figuresrepresents less than 0.1% of the fiber volume: 10 km of this fibercontains approximately 30 g of glass. The core-to-cladding diameterratio was approximately 1:3. This ratio can be controlled by changingthe thicknesses of the discs in the billet and the pressure appliedduring extrusion. Reducing this ratio, however, reduces the yield of theuseful preform length. The built-in polymer jacket facilitated the fiberdrawing compared to bare-glass-fiber drawing and helped avoidoxidization of the ChG. The fiber transmission losses (for a GGP fiberwith As₂Se₃ core) evaluated by cutback measurements were approximately10.9 dB/m at 1.55 μm (using a laser diode) and approximately 7.8 dB/matat 2 μm (using a Tm-doped fiber laser). The loss at the moment waslimited by the purity of the glass.

Fibers comprising a ChG core and a polymer jacket without anintermediate cladding were also formed using an extrusion process. ChGrods were prepared from commercial glass (AMI, Inc.) by melt-quenching,and polymer rods were prepared by thin-film processing. A ChG-core (G1),polymer-jacket (PES) preform was formed (FIGS. 6(b) and (c)) using anested billet (FIG. 6(a)) that comprised a ChG rod (11-mm diameter,60-mm length) provided within a polymer tube (46-mm outer diameter,140-mm length). This structure is referred to as GP. The drawn fiber isillustrated in FIGS. 6(d) and 6(e). In particular, a reflection opticalmicrograph of the fiber cross-section is shown in FIG. 6(d) and the coreis shown in the detail view of FIG. 6(e).

The robustness of the above-described ChG fibers is illustrated in FIG.7. The transmission at 1.55 μm over time was measured after bending a GPfiber for 10-minute intervals. There was no change for 1-inch benddiameters and larger, and no plastic memory was observed in knots withdiameters larger than 1 inch (FIG. 7(a)). Results for a 0.5-inch benddiameter are plotted in FIG. 7(b). The transmission did not decreaseafter an hour by more than 10% (5% on average). The effect of axialstress on optical transmission was also investigated. A 2-kg weight washung from multiple 5-cm-long GP fiber sections (FIG. 7(c)) for 18 hourseach and the transmission at 1.55 μm was measured. The transmission wasunaffected in this experiment. The fiber thus withstood 14.6 Kpsi (˜25.5MPa) with no change in its performance over this extended period oftime. This sets a lower limit on the fiber strength. Although the ChGdiameter was only 10 μm, the polymer jacket nevertheless enabledconvenient handling and reduced ageing of the fibers. Therefore, theoptical properties of the fiber are determined by the ChG, while themechanical properties are determined by the polymer. Separating thefunctionalities in this manner enables them to be independentlyoptimized.

A unique advantage of the thermally-compatible built-in polymer jacketis that it provides a mechanical scaffold for producing robust taperswithout first removing the polymer. ChG nano-tapers combine high opticalnonlinearities with dispersion control but are hampered by their extremefragility. The robustness of multi-material tapers is highlighted inFIG. 7(d), which shows a taper with a core diameter of 25 nm and anouter diameter of 3 μm.

The GP and GGP tapers were characterized in three ways after cutting thetaper at its center where the diameter is smallest (d_(min)=1.4 μm forthe core in both tapers). First, the structure was determined using SEMimaging, confirming that size reduction occurred at the same ratethroughout the cross-section during tapering (FIGS. 8(a) and (d)).Second, the ChG-polymer interface was determined by transmitting whitelight (coupled from the untapered end, d_(o)=10 μm) because the polymeris transparent in the visible and the ChG is not (FIGS. 4(b) and (e)).Third, the modal structure was determined by transmitting 1.55-μm CWlight (from a laser diode) through the core (FIGS. 4(c) and (f)). Themode was confined to the glass in the GGP taper due to the highcore-cladding index contrast and extended into the polymer in the GPtaper. The mechanical strength of the polymer jacket is thus harnessedwithout compromising the optical functionality of the taper.

The transmission in tapers of the same length (2.1 cm) with d_(min) from10 μm down to 400 nm for GGP and GP tapers was measured and the resultswere compared in FIG. 9 to calculations performed using the scalar beampropagation method in OptiBPM 10.0 (Optiwave). The transverse simulationwindow was 50×50 μm² and the step size was 0.1 μm in all threedimensions with perfectly matched layer boundary condition on all sides.The measured taper profile and realistic material losses were used. Thehigher loss in the GP taper was due to absorption of the evanescent tailin the polymer, while in the GGP taper the light remained confined tothe ChG. The measured losses were higher than predicted but this wasattributed to scatterers in the glass whose impact increases at smallerdiameters, deformations in the cross-section during tapering, andsurface roughness at the interfaces (particularly for the GP taper).

In conclusion, a novel one-step multi-material preform extrusion processhas been developed that produces hybrid ChG/polymer preforms that aredrawn into robust infrared fibers and tapers. The process helps obviatethe mechanical limitations of ChG fibers and enables optimizing theoptical properties for nonlinear applications.

We claim:
 1. A billet or preform, comprising: a first chalcogenide glassmass; a second chalcogenide glass mass; and a polymer mass having anaxial void, wherein the first and second chalcogenide glass masses arearranged in a non-overlapping stacked configuration, further wherein thefirst and second stacked chalcogenide glass masses are disposed withinthe void of the polymer mass.
 2. The billet of claim 1, wherein thefirst chalcogenide glass mass is one of As₂Se₃ and As₂Se_(1.5)S_(1.5).3. The billet of claim 1, wherein the second chalcogenide glass isAs₂S₃.
 4. The billet of claim 1, wherein the polymer is one ofpolyethersulfone or polyetherimide.